First let me say that I came across this site out of curiosity. I happen to feel extremely confident that the XBOX 360 product is excellent, especially compared to what the PS3 will be. Also, I do not think hacking into the XBOX 360 is necessarily good, since it may be illegal and/or wrong.
That said, in the interest of research, here are some thoughts on what I have read on this site.
First, there are many posts concerning the fact that mean CPU time required to determine Microsoft's private XBOX 360 key is comparable to the universe lifetime. Sporadically, these are followed by somebody pointing out that it would be much easier to get that key from hanging around Microsoft's XBOX headquarters, wherever that is.
My reaction to this is to point out that it would be even easier to buy lunch for an XBOX game programmer, of which there are thousands, and convince him or her to put in an Easter egg into their official release, or a live update of one of their official releases. Such eggs are not easily found by somebody whose job is to sign executables, especially with today's game deadline pressures. The egg might have the particular code you would want to run, such as a generic run of a non-signed executable, if that is possible and/or in anyone's hacking amusement interest.
Second, if one's objective is to use the 360 to run another OS that runs non-signed executables, in my opinion worrying about the signed code issues is a waste. The hypervisor should not matter either to such a person. The goal of such a person, in my opinion, should be to run different bios that does not check for signed code in the first place. This again may be a much easier (yet still very difficult) task.
Best of luck with the interesting research,